Internet Explorer allows users to set the default font size by going to View > Text Size, with "medium" being the typical 16px size.
Most other browsers also have a similar setting, although it may be hidden in various menus.
I recommend using relative font sizes so that users can easily adjust the text size based on their preferences. This way, those with poor eyesight can increase the font size for better readability, while others with high-resolution screens can reduce the font size to fit more content on the screen.
Keep in mind that when using relative font sizes, other elements on the screen may need to be sized in em's instead of px's or adapted to adjust with the text size. While fixed font sizes may seem easier initially, they can lead to issues like requiring horizontal scrolling on smaller screens.
Personally, I find it frustrating when websites have fixed widths that are wider than my display, forcing me to constantly scroll back and forth. Everyone's screen size is not the same, so assuming that all users have large monitors is not practical.